The present invention relates to a golf ball material which is useful as a golf ball component. More specifically, the invention relates to a golf ball material having an excellent softness, durability, rebound resilience and processability, and to a golf ball in which a molding of the golf ball material serves as a ball component.
Ionomer resins (ionomers) are useful materials in golf balls, among other things. In particular, terpolymer ionomers obtained by using metal cations to neutralize terpolymers of an α-olefin such as ethylene, an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, and an ester (softening monomer) of any of these unsaturated carboxylic acids are useful as materials capable of imparting softness to golf balls.
Golf balls manufactured by using ionomer materials generally have improved rebound resilience, toughness, and durability when compared with a balata ball construction. Ionomers are thus structurally important materials in golf balls. Ionomer resins such as Surlyn® available from DuPont and Iotek® available from Exxon-Mobil Chemical have become the materials of choice over traditional balata (trans polyisoprene, natural or synthetic) rubbers. However, ionomers having a high durability generally tend to be very hard. When used as golf ball materials, the resulting balls lack the softness required to impart the spin necessary to control the ball in play and also tend to have a poor feel on impact.
Yet, there has been a strong desire among golfers for a golf ball having properties that combine the good impact resistance and distance of hard copolymer ionomers such as ethylene-(meth)acrylic acid copolymers with the soft feel and spin provided by balata.
Terpolymer ionomers obtained by neutralizing with metal cations a terpolymer composed of an α-olefin such as ethylene, an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, and an unsaturated carboxylic acid ester (softening monomer) can provide softness. However, when used by themselves, such terpolymer ionomers are not suitable as golf ball materials because they have a low rebound resilience.
As related in a number of published U.S. patent publications (Patent Documents 1 to 3 below), melt blends of hard ionomer resins and soft ionomer resins have hitherto been investigated in the prior art in order to prepare ionomer materials which provide softness, high rebound resilience, high durability and also spin control. However, compared with hard copolymer ionomers by themselves, blends which also contain a soft ionomer resin tend to have a poor scuff resistance when used as golf ball covers. In addition, a number of U.S. patent publications (Patent Documents 4 to 7 below) describe materials obtained by using a high-molecular-weight highly neutralized ionomer together with a low-molecular-weight monofunctional higher aliphatic acid or a metal salt thereof so as to improve scuff resistance. However, in contrast with a high-molecular-weight ionomer component used by itself, incorporating a large amount of a low-molecular-weight higher aliphatic acid metal salt gives rise to incompatibility problems. As a result, when such blends are used in certain layers of a golf ball, the golf ball shows a poor durability. Also, when use is made of a monofunctional higher aliphatic acid which is not in the form of a metallic salt thereof, it is necessary to use a large amount of a cation source to neutralize the aliphatic acid. Moreover, in terms of the process, severe neutralization reaction conditions are required to carry out a high degree of neutralization. Furthermore, the resulting material has a lower melt flow rate (lower flowability), which adversely affects processability when it is injection-molded to make a golf ball. In addition, depending on the type of monofunctional higher aliphatic acid used, unfavorable odors are released, causing environmental pollution. Finally, the use of a higher aliphatic acid metal salt rather than a higher aliphatic acid result in higher costs, which is industrially disadvantageous.
As illustrated in several U.S. patent publications (Patent Documents 8 to 10 below), polyurethanes have been developed as golf ball materials. Polyurethanes, when used as golf ball components, especially the cover, are materials which provide a combination of good scuff resistance, spin control and soft feel on impact. However, since polyurethanes have a low rebound resilience, the performance of the golf ball deteriorates when used as a cover layer. Moreover, thermoset polyurethanes are more difficult to process than thermoplastic ionomer resins and cannot be recycled, increasing the material costs.
Also, a U.S. patent publication (Patent Document 11 below) discloses that highly neutralized ionomer resin materials containing an aliphatic acid or aliphatic acid derivative having a molecular weight of at least 280 have a good thermal stability, flowability and moldability. Yet, there remains room for improvement in terms of softness, rebound resilience, durability and the like.
Patent Document 1: U.S. Pat. No. 4,884,814
Patent Document 2: U.S. Pat. No. 5,120,791
Patent Document 3: U.S. Pat. No. 5,971,871
Patent Document 4: U.S. Pat. No. 6,100,321
Patent Document 5: U.S. Pat. No. 6,653,382
Patent Document 6: U.S. Pat. No. 6,777,472
Patent Document 7: U.S. Pat. No. 6,815,480
Patent Document 8: U.S. Pat. No. 6,974,854
Patent Document 9: U.S. Pat. No. 7,041,769
Patent Document 10: U.S. Pat. No. 7,090,798
Patent Document 11: U.S. Pat. No. 6,962,951